Transmission or.....
 

Alrighty....long time browser first time poster. Want to by thank everyone for the great threads and the indirect help. Now I need some direction.

1999 s10 blazer 4.3-4wd.... just removed pan, and replaced fluid and filter (redline). (killing myself now for not getting a drain plug put on the pan) anyways.....

it responded very well to the new fluid. (old fluid looked fine just waaaay tooo much of it) first couple of gears are really smooth and crisp. However when I am on the freeway...and I give the vehicle slight throttle input to "keep with traffic flow" it kind of "falls on its face slightly" ...rpms dip and rev about 200-400 or it try's to shift to overdrive (can't decide what it feels it's trying to do yet) if you give more than 20% throttle input it goes just fine... so RPM's are a little erratic when trying to cruse at frwy speed under throttle...closed throttle no rpm dip. Drops into lower gears just fine. Any thoughts? If I can be anymore confusing let me know :)

doesn't appear to do this an any other gears....tried to disconnecting the bat...thought cpu could iron out the problem on its own....didn't help..maybe even made it more pronounced.

113k, oem wheel and tire combo, new hubs (thanx to this forum), new shocks, recent minor tune up. Needs a cat and have weird intermittent evap system check light.

Thanks in advance.